Hitherto, coil components such as an inductor, a transformer, and a choke coil, have been used in various articles such as household electric appliances, industrial equipment, and vehicles. A coil component includes a magnetic core and a coil wound around the magnetic core. In this magnetic core, ferrite, which is excellent in magnetic property, shape flexibility and costs, has widely been used.
In recent years, a decrease in the size of power source devices of electronic instruments and others has been advancing, so that intense desires have been increased for coil components which are small in size and height, and are usable against a large current. As a result, the adoption of powder magnetic cores, in each of which a metallic magnetic powder is used, and which are higher in saturation magnetic flux density than ferrite, has been advancing. As metallic magnetic powders, for example, pure Fe particles, and Fe-based magnetic alloy particles such as those of Fe—Si-based, Fe—Al—Si-based and Fe—Cr—Si-based alloys are used.
The saturation magnetic flux density of a Fe-based magnetic alloy is, for example, 1 T or more, and a magnetic core using the Fe-based magnetic alloy has an excellent superimposing characteristic even when downsized. On the other hand, since such a magnetic core contains a large amount of Fe, and therefore easily gets rusty, and also has a low specific resistance and a large eddy current loss, the magnetic core is considered to be difficult to use in high-frequency applications with a frequency of more than 100 kHz unless the alloy particles are coated with an insulating material such as resin and glass. Accordingly, Fe-based magnetic alloy particles are bonded together via the insulating material, and therefore may be inferior in strength as compared to ferrite magnetic cores because the strength of the magnetic core is affected by the strength of the insulating material.
For improving the specific resistance and strength without subjecting alloy particles to an insulating treatment such as glass coating, Patent Document 1 discloses a magnetic core in which Fe—Si alloy powder particles containing 3.0 to 7.0% of Si and 0.02% or less of C with the balance being constituted by Fe are bonded together by an oxide mainly composed of Fe. In this document, Fe—Si alloy powder particles are formed into a compact, and the compact is held at 500° C. to 600° C. in heated water vapor, so that iron is reacted with the water vapor to form an oxide film which binds Fe—Si alloy powder particles together. The oxide film has a thick layer of Fe3O4 on the surface, and a layer of a mixture of Fe2O3, iron silicate and SiO2 on the inside thereof. The oxide film is composed of a substance having a high specific resistance, so that the specific resistance of the magnetic core is increased, and the strength of the magnetic core is secured.
Patent Document 2 discloses a magnetic core produced in the following manner: an alloy powder having a surface oxide film of 100 nm or less and mainly composed of Fe, Al and Si is heat-treated in an oxidizing atmosphere to further form an oxide layer of alumina at a location where the surface oxide film is broken during pressing, so that insulation between alloy powders is secured to reduce the eddy current loss.
Patent Document 3 discloses a magnetic core produced in the following manner: a compact composed of a group of particles of a soft magnetic alloy containing Fe and Si, and Cr or Al, i.e. a metal element that is more oxidizable than Fe, is heat-treated at 400° C. to 900° C., and the particles are bonded together via an oxide layer formed by the heat treatment, so that the magnetic core has a specific resistance of 1×10−3 Ω·cm (1×10−1 Ω·m) or more and a three-point rupture stress of 1.0 kgf/mm2 (9.8 MPa) or more.
Patent Document 4 discloses a magnetic material produced in the following manner: a Fe—Cr—Al-based magnetic powder containing 1.0 to 30.0% by mass of Cr and 1.0 to 8.0% by mass of Al with the balance being substantially constituted by Fe is heat-treated in an oxidizing atmosphere at 800° C. or higher, so that an oxide film containing 20% by mass or more of alumina is self-generated on the surface, and further, the heat-treated powder is solidified and compacted by discharge plasma sintering in a vacuum chamber. The magnetic material is used in alternating magnetic fields in, for example, stators and rotators of motors.